Gas turbine engines include turbine sections comprising a plurality of blades or buckets mounted to the periphery of a rotor wheel or disc in closely, angularly spaced relation. The turbine blades project into the hot gas stream to convert the kinetic energy of this working fluid stream to rotational mechanical energy. To accommodate material growth and shrinkage due to variations in temperature and centrifugal forces, the buckets are typically provided with root sections of a "fir tree" configuration, which are captured in dovetail slots in the rotor disc periphery. During engine operation, vibrations are induced in the turbine buckets. If left unchecked, these vibrations can result in premature fatigue failures in the buckets.
To dissipate the energy of these vibrations and hence lower vibrational amplitude and associated stresses, it is common practice to dispose dampers between adjacent buckets in positions to act against surfaces of tangentially projecting bucket platforms. When the turbine section rotates, the dampers are pressed against the platform surfaces by centrifugal forces. As the buckets vibrate, the damper and platform surfaces slide on each other to produce frictional forces effective in substantially absorbing and thus dissipating much of the vibrational energy.
The vibratory motion of the buckets is complex, but may be considered as composed of two basic modes. One is the tangential mode, wherein the direction of vibration is circumferential, and the angular spacing between adjacent buckets varies. The other is a radial mode, wherein the relative radial positions of adjacent buckets vary. These vibratory modes translate into movements of the platform surfaces of adjacent buckets in phased relation resulting in variations in their angular relationships. It will be appreciated that, for the dampers to be effective, sliding engagements between the damper and platform surfaces must be maintained for both tangential and radial vibrational modes and any combinations thereof.
Vibration dampers of a variety of configurations have been proposed. Flanders U.S. Pat. No. 2,310,412 discloses both circular and wedge-shaped dampers. Circular dampers are also disclosed in Dodd et al. U.S. Pat. No. 4,917,574. Allen U.S. Pat. No. 1,554,614; Stahl U.S. Pat. No. 4,111,603 and Hendley et al. U.S. Pat. No. 4,872,812, also disclose wedge-shaped dampers. T-shaped dampers are disclosed in Hess et al. U.S. Pat. No. 4,101,246; Nelson U.S. Pat. No. 4,182,598 and Jones et al. U.S. Pat. No. 4,347,040. Even X-shaped dampers, as shown in Damlis U.S. Pat. No. 3,666,376.
Of these various vibration damper configurations, the wedge shape is probably more commonly used in current gas turbine engine designs. It is found, however, that the wedge-shaped dampers do not always achieve exact fits with the V-shaped goove-defining platform surfaces of adjacent buckets as their angular relationships vary during bucket vibration and also due to manufacturing tolerances. That is, the dampers rock or become tilted under centrifugal loading, such that one of the damper surfaces lifts off from its confronting platform surface. Consequently, effective energy dissipating sliding action is not achieved with these platform surfaces, leading to premature fatigue failure of the buckets.